1. Field of the Invention
The present invention relates to an optical rotating recording medium, a recording method, a recording apparatus, and a reproducing apparatus, more particularly a medium, method, and apparatuses using a 16/17 modulation method (16/17 conversion method) modulating 16 bits of digital data to 17 bits of digital data.
The present invention also relates to an optical recording/reproducing apparatus combining the above recording apparatus and reproduction apparatus.
2. Description of the Related Art
As optical rotating recording media used for recording and reproducing digital data including video data, and audio data, compact disks (CDs), Mini Discs (MDs), and digital versatile disks (DVDs) (including DVD-RAM etc.) are known. In this specification, optical disks or optical-magnetical (MO) disks are called as optical recording media or optical rotating recording media. Hitherto, CDs and DVDs have been widely used as package media and MDs as music recording media. Reproducing apparatuses for reproducing data from such media and recording apparatuses for recording data on such media are also widely used.
CDs which are the basis of these technologies, are applied technology enabling the maximum performance in read only optical disks. For example, the signal to be recorded thereon is a so-called run-length limited (2, 10) code (hereinafter referred to as an RLL (2, 10) code) wherein the number of consecutive "0"s is limited from 2 to 10. Such the RLL is an eight to 14 modulation (EFM) for converting 8-bit data to 14-bit codewords and inserting 3 bits between the respective codewords in CDs and MDs, and 8/16modulation for converting 8-bit data to 16-bit codewords in DVDs.
Recording data to be recorded on optical rotating recording media is recorded by the followings: (a) by a non-return to zero inverse (NRZI) modulation wherein "1"s of the codewords are converted to be the inverse of the recording data "1" and "0", "0" of the codeword is converted to non-inversion, and (b) a "1" and "0" of the recording data are linked with the existence of pits on a magneto-optical disk or two polarities of magnetization. Accordingly, when using the above NRZI modulation code, the inversion intervals of the recording data become, when assuming a bit length of one codeword is T, 3T at a minimum and 11T at a maximum.
On the other hand, 1T corresponds to 0.47Tb (Th is a data bit length) in EFM, while 1T corresponds to 0.5Tb in 8/16modulation.
When considering the fact that the codeword bit length is the detection window width itself at the time of data modulation, a code having relatively broad minimum inversion interval and a relatively short detection window width of about half the data bit length particularly exhibits its features when the code is applied to read only disks which are little noise and show a good reproduction signal quality.
In recent years, however, there have been increasing demands for recording a large amount of digital data including compressed video data, compressed audio data, etc. on an optical disk or a magneto-optical disk.
As such a magneto-optical disk, there is proposed, for example, the invention of a magneto-optical disk described in Japanese Patent Application No. 11-176029 entitled "Optical Recording Media and Disk Cartridge" filed by the present assignee on Jun. 22, 1999. Such the optical disk is very compact, high density, and large capacity of a diameter of 50 mm and a storage capacity of 2 GB or a diameter of 64 mm and a storage capacity of 4 GB. In these cases, a blue laser is used as laser diode.
In such cases, to store a large amount of data on a writeable magneto-optical disk, sometimes, it is obtained better results by narrowing the minimum inversion interval and the resultant widening the detection window width rather than by using the above code where the run-length is limited to (2, 10) and the resultant detection window is narrowed.
The 16/17 modulation system is known as a modulation format securing a wide detection window width. The 16/17 modulation system is for converting 16 bits of data to a 17-bit codeword, while the run-length is limited from 0 to 6 in many cases. The 16/17 modulation system is widely used for example in a magnetic recording field. Such technology is disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 9-27171, 10-322217, 11-162113, and 10-1324520.
It should be noted that the above 16/17 modulation is optimized to be applied to a magnetic recording field, typically a partial response class 4 (PR4). Namely, note was taken of the fact that the response characteristic of a recording/reproducing system in a magnetic recording field is a differential characteristic. The recording data is generated from codewords by an interleaved NRZI method shown in FIG. 1, an impulse response equalizes final transfer characteristics to the PR4 characteristics shown in FIG. 2, and furthermore the data is reproduced by the maximum likelihood decoding method such as a Viterbi algorithm.
Reference number 101 in FIG. 1 shows a delay element giving a 2-clock delay by a codeword bit clock, while reference number 102 shows a processor for addition in modulo 2, that is, for an exclusive OR operation.
In the above 16/17 modulation applied to magnetic recording, the number of consecutive "0"s restricted to be not more than a certain number even when extracting data from a codeword every other bit, corresponding to the case of reproducing data by the above maximum likelihood decoding method.
Summarizing the problems to be solved by the invention, when assuming application of 16/17 modulation which is basically applied to the above magnetic recording, will be applied to an optical rotating recording media such as an optical disk or magneto-optical disk, the following problems may be encountered.
The response characteristics of a recording/reproducing system for an optical rotating recording media does not have differential characteristics like with magnetic recording. Accordingly, when recording data on optical rotating recording media etc., recording data is generated from codewords by the NRZI method, the reproducing signal is simply discriminated to be binary a value by the threshold value at time of reproduction, the final transfer characteristic of the impulse response thereof is equalized to the characteristics called partial response (1,1) (hereinafter referred to as PR(1,1)) as shown in FIG. 3, and furthermore the data is reproduced by the maxim-m likelihood decoding method such as a Viterbi algorithm etc. In this case, the above limitation in magnetic recording that the number of consecutive "0"s be not more than a certain number even when extracting every other data from the codewords, becomes meaningless.
PR4 and PR(1,1) will be compared from another viewpoint. FIGS. 2 and 3 are views of on impulse responses when the time is indicated an abscissa. When the frequency is indicated on an abscissa for comparison of PR4 and PR(1,1), PR4 becomes a sine wave and PR(1,1) becomes a cosine wave. Namely, the characteristics of PR4 for the magnetic recording and P(1,1) for the optical rotating recording media are basically different on the characteristics.
In the optical rotating recording media of the PR(1,1), when "1" continues in a codeword, the reproduction signal becomes an intermediate value, so the generation of a synchronization clock by a phase locked loop (PLL) when reproducing data becomes difficult. Furthermore, the data error rate after decoding is largely affected on a maximum likelihood decoding method like a Viterbi algorithm.
The 16/17 modulation for the above magnetic recording does not suppress the low frequency components of the modulation wave. Thus, an offset fluctuation arises and there is a possibility that the bit error rate (BER) will become higher at the time of reproduction.
In addition, in an optical rotating recording media, level changes by low frequencies caused by changes in the reflectance, the birefringence of the disk substrate, etc. are sometimes superimposed on the reproduction signal. When using the threshold judgement and PR(1,1) characteristics, the fluctuation of the low frequency of the reproduction signal may occur the lowering of the BER, so the low frequency components are often removed from the reproduction signal by using a high-pass filter. However, when the original recording data includes a large amount of low frequency components, the removal of the same causes new level changes which result in the lowering the BER. The above 16/17 modulation adapted to magnetic recording is not taken into account on the affect by low frequency components included in the recording data, thus the modulation cannot be applied to optical rotating recording media.
When applying the 16/17 modulation system to optical rotating recording media, it is necessary to correctly recognize the punctuation of 17-bit codewords and if must be determined the synchronizing pattern meeting with the modulation system.